1. Field of the Invention
This invention relates to a driving circuit for an electrophoretic display device.
2. Description of the Related Art
An electrophretic display device provided with microcapsules including electrophoretic particles has been known as a kind of electronic paper (Refer to Japanese Patent Application Publication No. 2009-229832.). FIGS. 11A, 11B and 11C show a conventional driving circuit for the electrophoretic display device. A plurality of microcapsules 22 is interposed between a pixel electrode 20 and a common electrode 21 to form a pixel, as shown in the drawings. Although FIGS. 11A, 11B and 11C show a single pixel only, the electrophoretic display device has a display unit composed of a plurality of pixels that are structured similarly.
The microcapsule 22 is shaped like a grain in which a plurality of black particles (electrophoretic particles) 23, a plurality of white particles (electrophoretic particles) 24 and dispersion medium 25 are encapsulated, and has a particle diameter of approximately 30-50 μm, for example. An outer shell of the microcapsule 22 is formed using acrylic resin such as polymethyl methacrylate and polyethyl metacrylate or transparent polymer resin such as urea resin and gum arabic, for example.
The black particles 23 are polymer or colloid particles made of black pigment such as aniline black and carbon black, for example, and are charged negatively, for example. The white particles 24 are polymer or colloid particles made of white pigment such as titanium dioxide, zinc oxide and antimony trioxide, for example, and are charged positively, for example. The dispersion medium 25 is a liquid dispersing the black particles 23 and the white particles 24 inside the microcapsule 22, and is made of water or alcoholic solvent, for example.
The pixel electrode 20 is made of translucent conductive material such as ITO (Indium Tin Oxide). A drive voltage output circuit DRV0 outputs to the pixel electrode 20 a drive voltage corresponding to two bits of display data A0 and B0 that are inputted from outside. A common voltage Vcom is applied to the common electrode 21 that is made of metal such as aluminum, for example. The common voltage Vcom is assumed to be a ground voltage (0 V) in the following explanation.
The drive voltage output circuit DRV0 outputs +15 V as the drive voltage when the display data A0=H and B0=L (“H” denotes a high level and “L” denotes a low level.), as shown in FIG. 11A. Then, the black particles 23 converge on the pixel electrode 20 side in the microcapsule 22 while the white particles 24 converge on the common electrode 21 side in the microcapsule 22, since there is formed an electric field directed from the pixel electrode 20 to the common electrode 21. As a result, the pixel displays black.
The drive voltage output circuit DRV0 outputs −15 V as the drive voltage when the display data A0=L and B0=H, as shown in FIG. 11B. Then, contrary to the case shown in FIG. 11A, the white particles 24 converge on the pixel electrode 20 side in the microcapsule 22 while the black particles 23 converge on the common electrode 21 side in the microcapsule 22, since there is formed an electric field directed from the common electrode 21 to the pixel electrode 20. As a result, the pixel displays white.
The drive voltage output circuit DRV0 outputs 0 V as the drive voltage when the display data A0=L and B0=L, as shown in FIG. 11C. Because the common electrode 21 and the pixel electrode 20 are at the same electric potential, there is formed no electric field between the common electrode 21 and the pixel electrode 20. Therefore, the pixel keeps immediately preceding display that is white, for example. Inputting the display data A0=H and B0=H is prohibited.
As described above, since the display is switched by changing the direction of the electric field between the common electrode 21 and the pixel electrode 20 as shown in FIGS. 11A and 11B, and the display is maintained by eliminating the electric field after switching the display as shown in FIG. 11C with the conventional driving circuit for the electrophoretic display device, it is made possible to realize a low power consumption display device. Also, it has a feature that when the power supply of the drive voltage output circuit DRV0 is turned off, the display immediately before the turning off of the power supply is maintained.
Therefore, the power supply is turned off after the display on the display unit is cleared (making all pixels display black, for example), or in the state in which the display on the display unit is maintained.
However, when the power supply is lost (when the battery is pulled out, for example), clearing the display on the display unit is not performed and the display before losing the power supply is left unchanged. In that case, there is caused a problem that a wrong display such as displaying wrong time is left unchanged, for example.
This invention is directed to offering a driving circuit for an electrophoretic display device, which makes it possible that either the display on the display unit is to be cleared or maintained is set in advance, and that when the power supply is lost, the loss of the power supply is detected and either clearing or maintaining the display on the display unit is performed in accordance with the setting.